Transmitters and Receptors

Transmitter Release ( Fig. 8.2 and Table 8.1 )

In resting nerve terminals, synaptic vesicles accumulate near the active zones, where they are tethered to the presynaptic densities by strands of docking proteins and actin. With the arrival of action potentials, voltage-gated Ca ²⁺ channels located immediately adjacent to the active zone in the presynaptic membrane are opened, leading to instant flooding of the active zone with Ca ²⁺ ions. These ions interact with several proteins on both the vesicle and the active zone, leading to vesicle fusion with the presynaptic membrane and neurotransmitter release.

Vesicle fusion begins with the interaction of vesicle SNARE proteins (v-SNARE) with a pair of presynaptic membrane proteins (t-SNAREs) to form a tight complex. The necessary metabolic components that trigger vesicle fusion are embedded in the active zone. A critical protein for these components is a large, multidomain protein called RIM that binds to Rab3, a GTP-binding protein on synaptic vesicles. Other proteins are also required for vesicle fusion and the release of neurotransmitters. These synaptic components include synaptic vesicle proteins (e.g. synaptotagmin, synaptobrevin, synaptophysin, and synapsins), proteins associated with synaptic vesicles (e.g. amphiphysin, dynamin, and CaM kinases), synaptic plasma membrane proteins (e.g. syntaxins, neurexins, and SNAP-25), and associated cytosolic proteins (e.g. complexins, SNAPs, and NSF).

Many of the identified synaptic proteins have distinctive roles in the excitation–secretion coupling and synaptic vesicle recovery mechanisms underlying synaptic transmission. Intricate models of the molecular machinery responsible for excitation–secretion coupling and neurotransmission at the synapse have been developed through various techniques. These and other local protein responses to Ca ²⁺ entry are extremely rapid, and the time between Ca ²⁺ entry and transmitter expulsion is normally less than 1 ms. In the case of small synaptic vesicles such as those containing glutamate or γ-aminobutyric acid (GABA), single spikes are sufficient to yield some transmitter release. In the case of peptidergic neurons, impulse frequencies of 10 Hz or more are required to induce typically slow (delay of 50 ms or more) transmitter release from the large, dense-cored vesicles. Therefore, the amount of transmitter released from a neuron is not fixed but can be modified by both intrinsic and extrinsic modulatory processes, the most important being the availability of free Ca ²⁺ in the presynaptic terminal.

Target Cell Receptor Binding

Transmitter molecules bind with receptor protein molecules in the postsynaptic membrane. The two main categories of receptors are ionotropic and metabotropic . Each category contains some receptors whose activation leads to the opening of ion pores and others whose activation leads to their closure.

Ionotropic Receptors

Ionotropic receptors are characterised by the presence of an ion channel within each receptor macromolecule ( Fig. 8.3 ). The transmitter binds with its specific receptor facing the synaptic cleft, causing it to change its conformational state, normally opening a closed channel. Ionotropic receptor channels are said to be transmitter-gated , or ligand - gated , signifying their capacity to bind a transmitter molecule or a drug substitute. As soon as the transmitter dissociates from the receptor or is broken down, the channel reverts to its original state (closes).

In Fig. 8.3A , the excitatory channel has been opened by the transmitter, causing a major influx of sodium ions (Na ⁺ ) and a minor efflux of potassium ions (K ⁺ ); the net result is an excitatory postsynaptic potential (EPSP) that depolarises the membrane. A larger depolarisation can be achieved by opening multiple transmitter-gated channels, resulting in summation and possibly reaching the threshold value to trigger an action potential. In Fig. 8.3B , the EPSP is followed immediately by an inhibitory postsynaptic potential (IPSP) that hyperpolarises the membrane to −70 mV, the chloride (Cl ⁻ ) equilibrium potential. A larger hyperpolarisation can be achieved by opening a K ⁺ channel, which has an equilibrium potential of −80 mV.

Ionotropic receptors are called fast receptors because of their immediate but brief effects on ion channels and their changes on the membrane potential.

Metabotropic receptors

Metabotropic receptors are so called because many can generate multiple metabolic effects within the cytoplasm of the neuron. The receptor macromolecule is a transmembrane protein devoid of an ion channel. Its function is initiated by the binding of a transmitter to its extracellular receptor site. This causes a change in the conformational state of the protein, activating one of the attached subunits (α or β subunit) that detaches and moves along the intracellular surface of the membrane. The subunits are called G proteins , because most bind with guanine nucleotides such as guanine triphosphate (GTP) or guanine diphosphate (GDP) . Their action on ion channels is usually indirect, via a second messenger system . However, some G proteins activate ion channels directly (see later).

A G protein with a stimulatory effect is known as G _s protein , whereas that with an inhibitory effect is known as G _i protein . Because of the numerous steps involved, metabotropic receptors are generally slow receptors. Membrane channel effects may continue for hundreds of milliseconds after a single stimulus. Additionally, the creation of intracellular second messengers can alter the excitability properties of neurons.

Three second messenger systems are well recognised:

• 1.

  The cyclic adenosine monophosphate (cAMP) system , responsible for phosphorylation of proteins.

• 2.

  The phosphoinositol system , responsible for liberating Ca ²⁺ from cytoplasmic stores.

• 3.

  The arachidonic acid system , responsible for production of arachidonic acid metabolites.

cAMP system

In the examples shown in Fig. 8.4 , transmitter–receptor binding (1) releases the α subunit (2) of a G _s protein, leaving it free to link with GTP (3), which in turn facilitates adenylate cyclase to convert adenosine triphosphate (ATP) to cAMP (4). The newly formed cAMP serves as the second messenger. Protein kinase A (PKA) in the membrane is stimulated by cAMP (5) to transfer phosphate ions from ATP to an ion channel (6), causing its pore to open and to allow Na ⁺ ions to enter, thus initiating depolarisation of the target neuron. When the G _s protein is switched off, the membrane-attached enzyme protein phosphatase (7) catalyses the extraction of phosphate ions, resulting in pore closure.

Phosphoinositol system

In the example shown in Fig. 8.5 , activation (1) of another kind of G _s protein α subunit (2) causes the effector enzyme phospholipase C (PLC) to split a membrane phospholipid (PIP ₂ ) (3) into a pair of second messengers: diacylglycerol (DAG) and inositol triphosphate (IP ₃ ). DAG activates protein kinase C (PKC) (4), which initiates protein phosphorylation (5). IP ₃ diffuses into the cytosol (6), where it opens Ca ²⁺ -gated channels, mainly in nearby membranes of smooth endoplasmic reticulum (7). The Ca ²⁺ ions activate certain Ca ²⁺ -dependent enzymes downstream, resulting in opening and/or closing of ion channels and possibly crossing the nuclear envelope to alter gene expression and protein production (8) (see ‘gene transcription effects’ below).

Arachidonic acid system

This is described later, in connection with histamine.

Gene transcription effects

It is also well established that the reflex responses to repetitive stimuli may be either progressively increased (a state of sensitisation , usually induced by noxious stimuli) or diminished (a state of habituation , induced by harmless stimuli). Animal experiments that investigate reflex arcs (involving sets of sensory neurons, motor neurons, and interneurons) have shown that a characteristic feature of sensitisation is the development of additional synaptic contacts between the interneurons and the motor neurons with an increase in transmitter synthesis and release, whereas in contrast, a characteristic feature of habituation is a reduction of transmitter synthesis and release. All these changes result from alterations of gene transcription. Repetitive noxious stimuli cause cAMP to increase its normal rate of activation of protein kinases involved in the phosphorylation of proteins that regulate gene transcription. This in turn results in an increased production of proteins (including enzymes) required for transmitter synthesis as well as other proteins for construction of additional channels and synaptic cytoskeletons. Repetitive harmless stimuli merely reduce the rate of transmitter synthesis and release. Both sensitisation and habituation can produce long-term or even permanent changes in how the nervous system responds to these stimuli in the future.

Gene transcription effects are especially important for forming long-term memories ( Chapter 33 ).

Fate of Neurotransmitters

The fate of transmitters released into synaptic clefts is highly variable. Some transmitters are inactivated within the cleft, some diffuse away into the cerebrospinal fluid via the extracellular fluid, and some are recycled either by direct uptake by the pre- or postsynaptic cells, or indirectly via glial cells.

The principal transmitters and modulators are shown in Table 8.2 . Respective receptor types are listed in Table 8.3 .

Amino Acid Transmitters

The most prevalent excitatory transmitter in the brain and spinal cord is the amino acid L-glutamate ( Fig. 8.7 ). As an important example, all neurons projecting into the white matter from the cerebral cortex, regardless of their destinations in other areas of the cortex, brainstem, or spinal cord, are excitatory and use glutamate as a transmitter. Glutamate is derived from α-ketoglutarate; it also provides the substrate for formation of the most common inhibitory transmitter, GABA.

GABA is widely distributed in the brain and spinal cord and is the transmitter in approximately one third of all synapses. Millions of GABAergic neurons form the bulk of the caudate and lentiform nuclei, and they are also concentrated in the hypothalamus, periaqueductal grey matter, and hippocampus. Moreover, GABA is the transmitter for the large Purkinje cells, which are the only output cells of the cerebellar cortex, projecting to the dentate and other cerebellar nuclei. GABA is synthesised from glutamate by the enzyme glutamic acid decarboxylase.

A third amino acid transmitter, glycine , is the same molecule that is used in the synthesis of proteins in all tissues. It is the simplest of the amino acids, being synthesised from glucose via serine. It is an inhibitory transmitter largely confined to interneurons of the brainstem and spinal cord.